A method for recalibrating an automatic parking brake in a motor vehicle is known from DE 10 2011 004 763 A1. Recalibration is performed when a fault occurs during a release process of the parking brake. A fault can result, for example, from a voltage dip in the on-board power system of the motor vehicle. A fault can also occur when the driver attempts to drive away even though the brake is still not (fully enough) opened and the release process consequently is not terminated. In the case of a fault, the brake motor is usually firstly switched off. If appropriate, a hydraulic assistance is requested and the brake motor is subsequently started again in order to continue the triggering process of the brake piston before starting the actual recalibration, to such an extent until the clamping force generated by the parking brake is eliminated and the parking brake consequently operates free of loading in the no-load mode.
In addition to the abovementioned cases, the recalibration is also started when a release process of the automatic parking brake runs in a load-free fashion. A load-free release process occurs when the motor current drops to its no-load level directly after the switch-on peak. The release process is aborted in this case because the position of the spindle nut can no longer be determined reliably. The recalibration process runs in two steps after the aborting of the release process. Firstly, the brake linings are moved to the brake disks by means of corresponding activation of the brake motor. The application of the brake linings to the brake disks is detected by a rise in the motor current. Subsequent to this, the automatic parking brake is opened in a manner known per se. The recalibration process is therefore ended. After the recalibration, the automatic parking brake is therefore in the open state and the closing process can be started again manually or automatically.
It is disadvantageous with the known recalibration methods in the case of a load-free release process that, at the time of checking, referred to below as a predetermined time, as to whether the motor current is in the load-free state, only one motor current which is present at this predetermined time is compared with an assumed threshold value in order to carry out the check. The current which is picked up by the brake motor is therefore measured after a predetermined time after the start of the release process and compared with the threshold value. If the measured current is below the threshold value at the predetermined time, a load-free release is assumed and the recalibration is started. However, this fails to take into account that the resulting current profile results from a superimposition of the switch-on current surge and the force elimination curve. The force elimination curve is also influenced substantially by a hydraulic admission pressure which is generated by the driver. Furthermore, the length of the switch-on current surge of the motor depends on a plurality of parameters, in particular the temperature, the mechanical time constants of the motor and cabling. It is therefore possible, for example by means of a large time constant of the brake motor and a high motor voltage, for incorrect interpretation of a motor current switch-on peak as a force elimination curve even though a release occurs without the elimination of force. In order to be able to reliably detect the state of the load-free release at the predetermined time even in such situations, the threshold value must be correspondingly raised, as a result of which unnecessary recalibration processes are also partially triggered.
The profile of the motor current after the renewed switching on of the brake motor can therefore occur in different ways depending on the operating state of the parking brake system. Hitherto, the operating state of the parking brake system during the triggering of the recalibration has not been taken into account with the result that unnecessary recalibration processes are frequently incurred. In the prior art, the selected threshold value consequently constitutes a compromise between the detection of a load-free release state of the parking brake and unnecessary starting of a recalibration process of the parking brake.